Band filter using film bulk acoustic resonator and method of fabricating the same

ABSTRACT

A band filter using a film bulk acoustic resonator and a method of fabricating the same. The method includes the steps of forming a membrane layer on a substrate, forming a plurality of resonators on an upper surface of the membrane layer, depositing a mask layer on a lower surface of the membrane layer and patterning the mask layer to form a plurality of main windows and sub windows, and forming cavities along the main windows in the substrate and forming sub walls in the cavities in such a way that the sub walls are separated apart from the membrane layer by using the notch effect caused during a dry etching. It is possible to precisely form cavities with desired sizes even if the cavities have different sizes, to reduce the notched areas in the cavities, to reduce the total size of the filter by decreasing a distance between the cavities and to reduce the total length of wires.

CROSS-REFERENCE TO RELATED APPLICATIONS.

This application claims priority from Korean Patent Application No.10-2005-0006771, filed on Jan. 25, 2005, the entire content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Apparatuses and methods consistent with the present invention relate toa band filter using a film bulk acoustic resonator and a method offabricating the same, and more particularly to a band filter using afilm bulk acoustic resonator in which a structure of a cavity forvibrating a thin film is modified and a method of fabricating the same.

2. Description of the Related Art

Generally, a film bulk acoustic resonator (FBAR) is a filter using abulk acoustic wave of a piezoelectric layer. A size of a generalfrequency filter is proportional to a wavelength of an electromagneticwave in a usage frequency band. Therefore, the size of a generalfrequency filter using the electromagnetic wave is relatively large. Forexample, when the frequency of the electromagnetic wave is 1 GHz, thesize of a general frequency filter is approximately 30 cm, and when thefrequency of the electromagnetic wave is 300 GHZ, the size of a generalfrequency filter is approximately 1 mm. However, if the bulk acousticwave of, the piezoelectric layer is used, a wavelength of the bulkacoustic wave becomes less as a fraction ( 1/10,000) of a wavelength ofthe electromagnetic wave. According to this, the electromagnetic wave isconverted into the bulk acoustic wave by the piezoelectric layer, andthe size of the filter becomes less in proportion to the wavelength ofthe bulk acoustic wave. That is, since the size of the frequency filterusing the bulk acoustic wave is approximately several hundred microns,and a plurality of the frequency filters using the bulk acoustic wavecan be fabricated at one time by using a wafer, mass production of aband filter is possible.

FIG. 1 is a schematic plane view showing a duplexer filter with a filmbulk acoustic resonator in accordance with a related art.

Referring to FIG. 1, a duplexer filter 1 includes a substrate 3, atransmission-side film bulk acoustic filter 5 and a reception-side filmbulk acoustic filter 7, formed on the substrate 3.

The transmission-side film bulk acoustic filter 5 and the reception-sidefilm bulk acoustic filter 7 are constituted by film bulk acousticresonators 10 formed on the substrate 3 and connected in series orparallel.

FIG. 2 is a cross-sectional view showing a filter with cavities ofdifferent sizes, taken along line I-I′ shown in FIG. 1.

Referring to FIG. 2, a plurality of cavities 3 a, 3 b and 3 c withdifferent sizes are formed on a substrate 3 and a membrane layer 4 isapplied over the substrate 3 to cover the cavities 3 a, 3 b and 3 c.Resonators 10 are formed on the membrane layer 4 at positionscorresponding to the cavities 3 a, 3 b and 3 c. A first electrode 11, apiezoelectric layer 13 and a second electrode 15 stacked sequentiallyconstitutes a resonator 10.

A method of manufacturing the filter described above is explainedbriefly below.

FIG. 3A to FIG. 3C illustrate a method of fabricating a duplexer filterwith film bulk acoustic resonators and FIGS. 4A and 4B are views forexplaining the loading effect and the notches generated upon dry etchingin accordance with the related art.

Referring to FIG. 3A, a membrane layer 4 is deposited on a substrate 3and resonators 10 are created on the membrane layer 4.

Referring to FIG. 3B, a mask layer 9 is formed on the opposite surfaceof the substrate 3 with respect to the membrane layer 4 and patterned toproduce windows 9 a, 9 b and 9 c at positions where cavities 3 a, 3 band 3 c would be formed.

Referring to FIG. 3C, the substrate 3 is etched through the widows 9 a,9 b and 9 c using an inductively coupled plasma (ICP) etching equipmentso that the cavities 3 a, 3 b and 3 c are formed in the substrate 3,leaving walls 3 d every between of the cavities 3 a, 3 b and 3 c.

However, in accordance with the related art described above, in case ofsimultaneously etching the substrate 3 to form the cavities 3 a, 3 b and3 c with different sizes using the ICP etching equipment, it isdifficult to precisely produce the cavities 3 a, 3 b and 3 c withdesired sizes due to a loading effect and a lag effect.

The loading effect means that edge portions of the cavities 3 a, 3 b and3 c, denoted by alphabetical reference “L” in FIG. 4A remain not beingetched due to relatively low etch rate at the edge portions of thecavities 3 a, 3 b and 3 c compared to the etch rate of the centerportions of the cavities 3 a, 3 b and 3 c.

The lag effect is generated because etch rates for every cavity 3 a, 3 band 3 c are different depending on the sizes of the cavities 3 a, 3 band 3 c. That is, etch rates in larger cavities are relatively higherthan that in the small cavities. Accordingly, even if the etching issimultaneously performed under the same condition for every cavity 3 a,3 b and 3 c, the cavity 3 c will be larger than the desired size afteran etching process in a case where the size of the cavity 3 c is largerthan that of the others.

The notch effect means that the substrate 3 is over-etched at the bottomof the cavities, i.e. near the membrane layer 4, thereby forming notchesin the walls 3 d. The notch denoted by alphabetical reference “N” inFIG. 4B is created since ions generated during the etching process arebombarded to and reflected from the membrane layer 4 and the reflectedions etch the walls 3 d by bombarding the walls 3 d.

As described above, in a case where all the cavities 3 a, 3 b and 3 cwith different sizes are etched at the same time, it is difficult toform the cavities 3 a, 3 b and 3 c with desired sizes due to theloading, lag and notch effects. Further, even the membrane layer 4 canbe damaged due to a stress and walls 3 d could be eliminated.

SUMMARY OF THE INVENTION

The present invention has been developed in order to solve the above andother problems associated with the related art. An aspect of the presentinvention is to provide a band filter using a film bulk acousticresonator with cavities having desired precise sizes and with aphysically strong structure. Another aspect of the present invention isto provide a method of fabricating such a band filter.

According to an aspect of the present invention, there is provided aband filter using a film bulk acoustic resonator, including: asubstrate; a membrane layer formed on the substrate; a plurality ofresonators formed on the membrane layer; a plurality of cavities formedin the substrate at positions corresponding to the resonators; and subwalls formed in the cavities in a net format, having a distance betweenthe membrane layer and the top thereof.

According to another aspect of the present invention, there is provideda method of fabricating a band filter using a film bulk acousticresonator, including the steps of: forming a membrane layer on asubstrate; forming a plurality of resonators on an upper surface of themembrane layer; depositing a mask layer on a lower surface of themembrane layer and patterning the mask layer to form a plurality of mainwindows and sub windows; and forming cavities along the main windows inthe substrate and forming sub walls in the cavities in such a way thatthe sub walls are separated apart from the membrane layer.

The sub windows may be rectangular, circular or corner-roundedrectangular.

The main windows may come in different sizes and the sub windows may beidentical in their sizes.

The sub walls may be apart from the membrane layer by using the notcheffect which is caused as ions generated during a dry etching processare reflected from the membrane layer and the reflected ions etch aportion of the sub walls.

The thickness of the sub walls may be determined depending on the amountof etching caused by the reflected ions so as for the sub walls to beseparated from the membrane layer.

The membrane layer may be formed of a protective material which is notetched by the dry etching.

The membrane layer may be an oxide layer or a metal layer.

The substrate may be a silicon substrate.

The mask layer may be a photoresist layer or an oxide layer.

The oxide layer may be formed by a thermal oxidation process or a thinfilm deposition process.

The photoresist layer may be formed by a spin coasting process.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will be moreapparent by describing exemplary embodiments of the present inventionwith reference to the accompanying drawings, in which:

FIG. 1 is a schematic plane view of a duplexer filter with a film bulkacoustic resonator in accordance with a related art;

FIG. 2 is a cross-sectional view showing the filter in accordance withthe related art, taken along the line I-I shown in FIG. 1;

FIG. 3A to FIG. 3C illustrate a method of fabricating a duplexer filterwith a film bulk acoustic resonator in accordance with the related art;

FIG. 4A is a view for explaining a loading effect caused upon dryetching in accordance with the related art;

FIG. 4B is a view for explaining a notch effect caused upon dryingetching in accordance with the related art;

FIG. 5 is a cross-sectional view of a band filter using a film bulkresonator in accordance with an exemplary embodiment of the presentinvention;

FIG. 6A to FIG. 6D illustrate a method of fabricating a band filterusing a film acoustic resonator in accordance with an exemplaryembodiment of the present invention; and

FIG. 7A to FIG. 7C are plane views showing mask layer patterns used in amethod of fabricating a bank filter in accordance with an exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will be described ingreater detail with reference to the accompanying drawings.

FIG. 5 is a cross-sectional view showing a portion of a band filterusing a film bulk acoustic resonator in accordance with an exemplaryembodiment of the present invention.

Referring to FIG. 5, a band filter using a film bulk acoustic resonator110 includes a substrate 101, a membrane layer 103 and a resonator 110.

The resonator 110 includes a piezoelectric layer 111 being vibrated byconverting electrical signals to mechanical signals, an upper electrode113 disposed on the piezoelectric layer 111 and a lower electrode 115disposed under the piezoelectric layer 111.

The membrane layer 103 is a support layer for enabling the resonator 110to vibrate and serves as an etching stop layer to define the depth of anetching for cavities 101 _(a1), 101 _(a2) and 101 _(a3) upon etching toform the cavities 101 _(a1), 101 _(a2) and 101 _(a3).

The cavities 101 _(a1), 101 _(a2) and 101 _(a3) are formed in thesubstrate 101 provided with the resonators 110 at the positionscorresponding to the resonators 110. The sizes of the cavities 101_(a1), 101 _(a2) and 101 _(a3) are determined to correspond tocharacteristics of the resonators 110, so that the sizes of the cavities101 _(a1), 101 _(a2) and 101 _(a3) are different.

Each of the cavities 101 _(a1), 101 _(a2) and 101 _(a3) has sub walls101 _(b) formed in a net format. The top of the sub walls 101 _(b) areapart from the membrane layer 103 by a distance to provide an air gap Gbetween the membrane layer 103 and the sub walls 101 _(b) to let theresonators 110 to vibrate therein. The sub walls 101 _(b) can be apartfrom sidewalls, i.e. main walls, 101 _(d) of the cavities 101 _(a1), 101_(a2) and 101 _(a3). The sub walls 101 _(b) can serve as reinforcementmembers.

Next, a method of fabricating a band filter using a film bulk acousticresonator will be described below.

FIG. 6A to FIG. 6D illustrates a method of fabricating a band filter inaccordance with an exemplary embodiment of the present invention, andFIG. 7A to FIG. 7C illustrate mask layers with sub windows in accordancewith various examples, which are used in the method in accordance withembodiments of the present invention.

Referring to FIG. 6A, a substrate 101 is prepared and a membrane layer103 is deposited with a certain thickness on the upper surface of thesubstrate 101. The substrate 101 is, for example, a silicon substrateand the membrane layer 103 supports resonators 110 to be able to vibrateand defines the etching depth of cavities 101 _(a1), 101 _(a2) and 101_(a3) discussed below. Accordingly, the membrane 103 is preferablyformed of a protective material that will not be etched upon cavityetching.

The protective material is an SiO₂ layer or a metal layer. The silicondioxide layer can be formed by oxidizing the surface of a silicon waferwith a thermal oxidation process and by a thin film deposition process.The thin film may be deposited with a chemical vapor deposition (CVD)process or a physical vapor deposition (PVD) process. The metal layercan be formed of a chrome Cr layer or an aluminum nitride AlN layer withthe CVD or the PVD process.

Referring to FIG. 6B, the resonators 110 are formed on the membranelayer 103. For formation of the resonators 110, a lower electrode layer,a piezoelectric layer and an upper electrode layer, not shown, areformed sequentially, and patterned in order of the upper electrodelayer, the piezoelectric layer and the lower electrode layer so as toform sequentially upper electrodes 113, piezoelectric layer patterns111, and lower electrodes 115, thereby forming the resonators 110. Theresonators 110 are connected via wires in series or in parallel witheach other (wires not illustrated in the drawings).

Referring to FIG. 6C and FIG. 7A to FIG. 7C, a mask layer 107 is formedon the lower surface of the substrate 101 provided with the resonators110, and patterned to form windows (main widows) 107 _(a1), 107 _(a2)and 107 _(a3), which are used for forming cavities 101 _(a1), 101 _(a2)and 101 _(a3), and sub windows 107 _(b). The mask layer 107 is a silicondioxide layer or a photoresist layer. In a case where the mask layer isa silicon oxide layer, the mask layer and the membrane layer can besimultaneously processed.

Sizes of the main windows 107 _(a1), 107 _(a2) and 107 _(a3) are set tobe different to make characteristics of the resonators 110 be different.The sub windows 107 _(b) are formed in each main window 107 _(a1), 107_(a2) and 107 _(a3). Referring to FIG. 7A to FIG. 7C, the sub windowscan be a rectangular, a corner-rounded rectangular or a circle. Inconsideration of a dry etching characteristic that an etching rate islower at corners compared to the center of an object shape to be formed,it is preferable that the sub windows 107 _(b) are circular orcorner-rounded rectangular.

The sub windows 107 _(b) are identical in a size to make the etchingcondition be identical for every window even if the sizes of thecavities 101 _(a1), 101 _(a2) and 101 _(a3) are different. That is, evenif the cavities 101 _(a1), 101 _(a2) and 101 _(a3) have different sizes,inside of the cavities 101 _(a1), 101 _(a2) and 101 _(a3) are etchedthrough the sub windows 107 b with the same size so that etchingcondition for each cavity becomes the same.

Referring to FIG. 6D, the substrate 101 is etched along the maskpatterns described above using an ICP etching equipment, thereby formingthe cavities 101 a and sub walls 101 _(b) in a net format. The sub walls101 _(b) have a gap between the membrane layer 103 due to the notcheffect, that is, the sub walls 101 _(b) are separated from the membranelayer 103 having an air gap G between both of them. The notch effect isdiscussed above but will be described in more detailed below.

First, in accordance with the principle of the dry etching, ifelectrical energy is applied to gas molecules in a vacuum chamber,valance electrons are pushed out of the outermost shell of the orbit ofa molecule or an atom due to the collision of accelerated electrons, sothat ions or radicals with high reactivity are produced. The ions andradicals are accelerated due to the continuous collision and van derWaals force and collided with the surface of a material, therebybreading a bond in molecules in several micrometers and cutting away thesubstrate in the sub windows 107 _(b). Meanwhile, after the etching isprogressed, the ions and radicals finally reach the membrane layer 103and are reflected from the surface of the membrane layer 103 serving asa dielectric. At this time, some ions and radicals of the reflected ionsand radicals are collided with the sub walls 107 _(b), thereby etchingaway a portion of the sub walls 107 _(b). Accordingly, the top of thesub walls 107 _(b) are separated from the membrane layer 103 by adistance. The thickness of the sub walls 103 are determined so as forthe top thereof to be completely separated from the membrane layer 103considering the amount of notch effect.

As described above, it is possible to reduce a notch area by using thesub windows 107 _(b). Accordingly, a distance D between cavities 101_(a1), 101 _(a2) and 101 _(a3) will be reduced. (See FIG. 5). Further,since the substrate is etched through the sub windows with the samesize, etching condition for each cavity 101 _(a1), 101 _(a2) and 101_(a3) becomes the same so that the cavities 101 _(a1), 101 _(a2) and 101_(a3) with desired sizes can be formed even though the cavities 101_(a1), 101 _(a2) and 101 _(a3) are different in sizes and etched at thesame time.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teaching can bereadily applied to other types of exemplary embodiments. Also, thedescription of the exemplary embodiments of the present invention areintended to be illustrative, and not to limit the scope of the claims,and many alternatives, modifications, and variations will be apparent tothose skilled in the art.

1. A method of fabricating a band filter using a film bulk acousticresonator, comprising: forming a membrane layer on an upper surface of asubstrate; forming a plurality of resonators on an upper surface of themembrane layer; depositing a mask layer on a lower surface of thesubstrate and patterning the mask layer to form a plurality of mainwindows and sub windows; and forming cavities to penetrate through thesubstrate along the main windows and the sub windows in the substrate atpositions corresponding to respective resonators and forming sub wallsin the cavities in such a way that the sub walls are separated apartfrom the membrane layer, wherein each of the cavities has the sub wallsarranged in a format.
 2. The method according to claim 1, wherein thesub windows are rectangular, circular or corner-rounded rectangular. 3.The method according to claim 1, wherein the main windows are formed indifferent sizes and the sub windows are formed in identical sizes. 4.The method according to claim 1, wherein the substrate is a siliconsubstrate.
 5. The method according to claim 1, wherein the formatcomprises a net format.
 6. The method according to claim 1, wherein thesub walls are separated apart from the membrane layer by using a notcheffect which is caused as ions generated during a dry etching processare reflected from the membrane layer and the reflected ions etch aportion of the sub walls.
 7. The method according to claim 6, wherein athickness of the sub walls is determined depending on an amount ofetching caused by the reflected ions so as for the sub walls to beseparated from the membrane layer.
 8. The method according to claim 1,wherein the membrane layer is formed of a protective material which isnot etched by dry etching.
 9. The method according to claim 8, whereinthe membrane layer is an oxide layer or a metal layer.
 10. The methodaccording to claim 1, wherein the mask layer is a photoresist layer oran oxide layer.
 11. The method according claim 10, wherein the oxidelayer is formed by a thermal oxidation process or a thin film depositionprocess.
 12. The method according to claim 10, wherein the photoresistlayer is formed by a spin coating process.